Methods of measuring regional cerebral blood flow (CBF) with PET and 0-15 water involve the use of two parameters, the flow and the partition coefficient (p) for water, which is defined as the ratio of the tissue and blood tracer concentrations at equilibrium. The value of p reflects the relative solubility of the tracer in tissue and blood and can be calculated as the ration of brain and blood water contents, using published.data for these values. In some methods for measuring CBF and PET, this calculated value is used in the mathematical formulation, and flow is calculated from measured blood and tissue radioactivity. In alternative methods, values for both flow and p are estimated from the measured data. With these methods, the value of p was found to be lower than that predicted from the ratio of brain and blood water contents. To address this issue further, a method was developed to measure p directly with PET and 0-15 water. IT involves constructing a radiotracer infusion schedule which rapidly produces and sustains equilibrium between tissue and blood. This method was used to measure p in baboons. Two injections of 0-15 water are required in each study: a blous i.v. injections from which the arterial impulse response is obtained, and an 8 min designed infusion administered by a computer-controlled pump. The value of p calculated as the ratio of the tissue to blood concentrations from 3 to 8 min was 0.80 ml/g (0.006 SD, n=10). In addition, p was estimated with a pixel-by-pixel least squares method from the full 8 min time course of brain and blood radioactivity data; values were 0.76-0.78 ml/g. These results suggest that the in vivo p as measured with PET within a short period following tracer administration is smaller than the value (0.98 for gray matter, 0.90 for whole brain) based on in vitro water contents of brain and blood. In order to further explore this discrepancy, we will measure the value of p in baboons with the above PET method.